Solenoid operated switch with dashpot



P 1956 A. c. GASCOINE ETAL 2,764,648

SOLENOID OPERATED SWITCH WITH DASHPOT Filed May 22, 1955 I71 vevz furs A 1C Gadc R A, F Chit/6 United States Patent SOLENOID OPERATED SWITCH DASHPOT Arthur Cecil Gascoine and Royston Azrthur Frederick Craven, Watford, England, assignors to Watford Electric .& .Manufacturing Company Limited, a British company, and Arthur Cecil Gascoine, both of Watford, England Application May v 22, 1953, Serial No. 356,815

1 Claim. (Cl. 200-97) This invention relates to dash-pots of the kindcomprising a piston reciprocating in a fluid-containing cylinder, and has for its object to provide a construction in which the operating characteristic is accurately reproducible using normal manufacturing procedure. This is particularly important when an operation has :to be delayed for along period, c. g. several minutes, by the dash-pot. One example is to be found in the overload protection of an alternating current motor by means of an overload cut out. 0n starting up the motor, the current may be several times the full load current, but the thermal capacity of the motor is such :that this is permissible for a period of say 5 or :minutes so :that the operation of the cut-out can safely be delayed for lthiS :period.

It is found in practice that, using ordinary manufacturing operations, a metal piston working in a brass cylinder filled with ;a viscous \oi'l does not :give easily reproducible results,

According to this invention there is provided a dashpot of the kind set forth wherein the piston is of metal and the part of the dash-pot cylinder which the piston moves during its operative movement is of glass. We have found such a dash-pot to give very satisfactory results in uniformity of operating characteristics. This may be considered surprising but in practice this is found to be the case, The explanation for this has not been definitely established; it is however, reasonable to assame, without prejudice to this invention, that one of the principal factors contributing to the success of the arrangement is the nature of the glass surface.

Preferably the cylinder is made entirely of glass. This may be either toughened glass or glass of the kind sold under the registered trademark Pyrexl The cylinder may be either drawn or moulded, and may subsequently have its internal surface ground. The piston may be of brass.

According to a feature of the invention the piston may be provided with one or more apertures of predetermined size to allow passage of a viscous dash-pot liquid there through. In the case where there are two or more apertures one of the apertures may be provided with a nonreturn valve.

According to another feature of the invention, when the piston engages a flat end wall of the cylinder, the piston and/or the end wall is provided with a protrusion or protrusions at a recess to prevent a flat surface of the piston coming into contact with said end wall, thereby reducing the possibility of sticking between the surfaces.

According to yet another feature of the invention the dash-pot liquid may be a viscous liquid silicone, as for example, that sold under the registered trademark Silicone DC. 200. The particular advantage of this liquid is that its change in viscosity with temperature is substantially less than the change which occurs in thick mineral oil.

One embodiment of the present invention will now be described merely by way of example with reference to the accompanying drawings, whereof:

Figure 1 is a side elevation, partly in section, of a delayed action overload cut-out for an alternating current motor incorporating a dash-pot in accordance with the present invention, and

Figure 2 is a plan view of the cut-out shown in Figure 1.

Referring to the drawings, a yoke 10 carries a pole piece 11, 'a plunger guide 12, and a bobbin 13 having a tubular core =14, and "carrying an energisin'g coil. "The pole piece 11 and the guide 12 enter the open ends of the core 14 and thereby "locatingthe bobbin '13.

'The pole-piece 11 has -a stepped axial bore 15 in which is mounted a headed operating rod 16. The rod 16 passes through .a hole 1-7 in a pivoted contact assemblage generally indicated at 18, and is threaded to carry nuts 1-9 which engage, and pivot the assemblage, when the rod 16 is moved axially. The assemblage 18 comprises a channel sectioned arm 21 which is pivoted as at'2 2 to brackets 23a (see Figure 2) carried by a contact base 23 supported from the yoke 10, The arm '21 carries two contact blades 27 (see Figure 2) which engage with a fixed contact 24 carried by the base '23. The contacts 27 are urged into engagement with contact 24 by a coiled compression spring .25 carried 'on the rod 16 between a step 26 iof'the bore 15 and the head 31 of the rod. In the drawings :the contacts 27 and 24 are shown in their engaged :position.

The :guide v12 "is cylindrical Zin cross-section and screwed thereon is a pair of co-operating locking rings 29, 30 between which is clamped :a rim .32 of :a glass dash pot cylinder generally indicated at V3? (see Figure l). The cylinder 33 has an end wall 34, a vertical cylindrical portion .35., and a truncated conical portion 36 diverging therefrom. lihe cylinder is made from glass sold :under the registered trademark Pyrex.

A headed plunger 40 made of mild steel is located within the guide 12 by a circumferential flange 42 which slides in the bore 44 of the guide, and it extends into the cylinder 33 where it carries .a brass piston 41 which has an internally threaded sleeve 45 which screws onto the "end -46 of the plunger. The piston '41 is a sliding fit .in the portion 35 of the cylinder 33. The sleeve 45 is provided with ports -47 which communicate with the space 48 between the end 46 of the plunger .12 and the back or upper side .of the piston 41, and :the piston .has an aperture 49 which places the space .50 in front of the piston in communication with the space 48. A ball .51 is carried. within the space 48 and seats on the mouth of the aperture 49, constituting with .the aperture a .nonreturn valve. The piston 41 also has three additional apertures such as 43 (see Figure 1) of different predetermined sizes which are spaced apart in a quadrant of the piston at equal radial distances from the centre thereof. The rear face 57 of the piston 41 carries a rotatable disc 58 concentrically therewith, and of a diameter slightly less than the piston. The disc 58 is provided with three apertures such as 52 (see Figure 1) which correspond in size to the apertures 43. The apertures 52 are so arranged that by rotation of the disc 58 any one of the apertures 52 may be aligned with its corresponding aperture 43, and, in this manner, any one of the three apertures 43 may be selected to place the space 50 in communication with the space at the back of the piston. The disc 58 is fixed in its selected position by a screw 54 which passes through the disc and is screwed into the back of the piston. The piston 41 and the disc 58 are made from brass, while the plunger 40 is made from mild steel.

The front face 55 of the piston 41 is provided with a number of protrusions 56 arranged in a ring around the aperture 49, to prevent the flat face 55 coming into contact with the end wall 34 and thereby to reduce the pos- 3 sibility of sticking between the surfaces of the face and the end wall when the cylinder contains a viscous dashpot fluid. The protrusions 56 may be carried on the end wall 34 if desired, and one, or a plurality thereof may be provided in either case.

The cylinder 33 is partially filled with a viscous liquid silicone, for example, that sold under the registered trademark Silicone D. C. 200 so that the piston 41 is always submerged.

In operation the pole-piece is energised by the loadcurrent to the alternating current motor which the cut-out is to control, and the contracts 22, 24 are placed in circuit with the motor. When the load current exceeds the safe predetermined value the magnetic flux induced in the pole-piece 11 draws the plunger 12, and therefore the piston 41 from the cylindrical portion 35 of the cylinder 33, the silicone liquid passing slowly through the selected aperture 43, 52 from behind the piston to the space 50 in front of the piston. When the piston reaches the mouth of the cylindrical portion 35, the drag on the piston is relieved, and the plunger 12 moves rapidly towards the pole piece 11, its head striking the head 31 of the rod 16 and driving the rod into the pole-piece thereby rocking the arm 21 and disengaging the contacts 27, 24 to cut out the motor. In the disengaged position of the contacts 27 24 a spring arm 60 (see Figure 2) engages in a necked portion 61 (see Figure 1) in the rod 16 and maintains the contacts open. In order to reset the contacts 27, 24 the arm 60 has a handle 67 (see Figure 2) by which it is moved out of engagement with the necked portion 61, the contacts then being brought into engagement under the action of the spring 25.

In the absence of magnetic flux induced in the polepiece 11, the plunger 12 and piston 41 return to the position shown in Figure 1 under the action of their own weight, the dash-pot liquid passing freely through the aperture 49 and the selected aperture 43, 52 from the front to the rear of the piston.

The selected aperture 43, 52 is chosen to give the required time delay to the actuation of the cut-out, and any convenient number of difierent size apertures 43, 52 may be provided to give a range of time delays at any one of which the cut-out may be set to operate.

A viscous liquid silicone is used as the dash-pot fluid together with a piston having a comparatively large aperture 43, S2 and the tendency for the displacement of the piston to vary with changes in the amount of clearance between the piston and the portion 35 of the cylinder, consequent on changes in temperature is diminished, as the amount of the liquid passing through the clearance is small compared with that passing through the aperture on the upward movement of the piston.

With the piston and the disc of brass, and the cylinder of Pyrex glass, this tendency is even further diminished since the thermal coefficient of expansion of the brass is approximately twice that of the glass, the decrease in viscosity of the silicone liquid With rise in temperature is counter-balanced by a decreased clearance between the piston and the portion 35 of the cylinder.

The cut-out as described has been found to give a reproducible time delay before actuation, which is not the case when the piston and cylinder of the dash-pot are made from metal using normal manufacturing procedure.

We claim:

A delay-action electrical circuit-breaker comprising in combination a yoke, an electro-magnet coil carried thereby, a pole-piece having a central axial bore through it fixed within said coil, an operating rod extending through and beyond said bore, a spring urging said rod inwards in said coil, a hollow guide carried on said yoke, a magnetisable plunger movable in said guide towards said operating rod to impinge thereon and move it, a fixed electrical contact carried by said yoke, a contact-carrying member normally engaging said fixed contact and operatively engaged with said operating rod and movable with it to separate said contacts, a manually releasable spring-controlled detent engageable with said operating rod after it has been moved by said plunger, a glass cylinder carried on said yoke, a brass piston having openings through it carried on said plunger and reciprocable in said cylinder, means for adjusting said openings through said piston, a silicone fluid in said cylinder, and means limiting the approach of the face of the piston to the end of said cylinder.

References Cited in the file of this patent UNITED STATES PATENTS Re. 16,551 Wilms Feb. 15, 1927 1,212,322 Buckingham Jan. 16, 1917 1,389,140 Hodgkins Aug. 30, 1921 1,604,669 Van Valkenburg Oct. 26, 1926 2,182,450 Clarkson Dec. 5, 1939 2,308,660 Kouyoumjian Jan. 19, 1943 2,443,338 Berry June 15, 1948 2,493,371 Vradenburgh Jan. 3, 1950 2,622,707 Faus Dec. 23, 1952 FOREIGN PATENTS 24,115 Great Britain Sept. 22, 1910 377,816 Great Britain Aug. 4, 1932 533,040 Great Britain Feb. 5, 1941 

